Calculation of redundant bend in multi-core fiber for safety

ABSTRACT

A fiber includes M primary cores and N redundant cores, where M an integer is greater than two and N is an integer greater than one. Interferometric circuitry detects interferometric pattern data associated with the M primary cores and the N redundant cores when the optical fiber is placed into a sensing position. Data processing circuitry calculates a primary core fiber bend value for the M primary cores and a redundant core fiber bend value for the N redundant cores based on a predetermined geometry of the M primary cores and the N redundant cores in the fiber and detected interferometric pattern data associated with the M primary cores and the N redundant cores. The primary core fiber bend value and the redundant core fiber bend value are compared in a comparison. The detected data for the M primary cores is determined reliable or unreliable based on the comparison. A signal is generated in response to an unreliable determination.

This application is the U.S. national phase of International ApplicationNo. PCT/US2017/038699 filed Jun. 22, 2017 which designated the U.S. andclaims priority and benefit of U.S. Provisional Patent Application62/359,716, filed Jul. 8, 2016, entitled “CALCULATION OF REDUNDANT BENDIN MULTI-CORE FIBER FOR SAFETY,” the entire contents of each of whichare hereby incorporated by reference.

BACKGROUND

Multi-core optical fiber may be used to determine the bend angles of anoptical fiber. A multi-core fiber having three cores can be used toseparate the deformation of the fiber into two bend angles (pitch andyaw) and fiber elongation (strain).

In fiber optic based sensing of bend angles, a multi-channel distributedstrain sensing system may be used to detect the change in strain foreach of several cores within a multi-core optical bend sensing fiber asdescribed for example in U.S. Pat. No. 8,773,650, incorporated herein byreference. Multiple distributed strain measurements may be combinedthrough a system of equations to produce a set of physical measurementsincluding curvature and axial strain as described in U.S. Pat. No.8,531,655, incorporated herein by reference. These physical measurementsmay be used to determine distributed bend angles of the optical fiber.

Some applications for bend sensing fiber require a high degree ofconfidence or safety in terms of the accuracy and reliability of thebend sensing output. An example application is robotic arms used insurgical or other environments. One way of improving the confidence andsafety in the operation of these robotic arms is to use redundancy. Forexample, rather than using just one position encoder to determine acurrent angular position of a robotic arm joint, two position encodersmay be used and the outputs compared to determine if both encoders aregenerating the same position or the same position with an allowedmargin. If the outputs do not agree within the allowed margin, then asafety or fault signal may be generated to alert an operator of theerror situation.

A redundant fiber may be an option for some bend sensing applications toensure safe and reliable bend sensing output information. But in othersituations, a redundant fiber may not be possible or desirable, e.g.,due to space constraints, cost constraints, etc. For example, insurgical robotic arms, the conduit space that contains the bend sensingfiber may be too tight to accommodate two fibers. What is needed isanother technological solution that ensures safe and reliable bendsensing output information but that does not require a redundant fiber.

SUMMARY

An interferometric measurement system measures an optical fiberincluding M primary cores and N redundant cores, where M is an integeris greater than two and N is an integer greater than one. (In oneexample implementation, M=3 and N=3, and in another, M=4 and N=2).Interferometric detection circuitry detects measurement interferometricpattern data associated with the M primary cores and the N redundantcores when the optical fiber is placed into a sensing position. Dataprocessing circuitry calculates, based on a predetermined geometry ofthe M primary cores and the N redundant cores and detected measurementinterferometric pattern data associated with the M primary cores and theN redundant cores, a primary core fiber bend value for the M primarycores and a redundant core fiber bend value for the N redundant cores.It also performs a comparison of the primary core fiber bend value andthe redundant core fiber bend value, determines whether the detectedmeasurement interferometric pattern data associated with the M primarycores is reliable or unreliable based on the comparison, and generates asignal in response to an unreliable determination.

In an example embodiment, the data processing circuitry determines thatthe detected measurement interferometric pattern data associated withthe M primary cores is unreliable when a difference between the primarycore fiber bend value and the redundant core fiber bend value exceeds apredetermined threshold.

The signal may represent one or more of the following: (a) an error inoperation of optical and/or electronic sensing and processing circuitry,(b) an error in a connection with the optical fiber, (c) an error incalibration data determined for the optical fiber, (d) an error causedby a force experienced by the fiber that is otherwise not compensatedfor by the data processing circuitry, or (e) an error in processing orexecution of algorithms resulting in miscalculation of the bend. Theerror can be a force caused by a pinch of the optical fiber and/orcaused by one or more environmental conditions. The signal can alsorepresent a reliability or an unreliability of the detected measurementinterferometric pattern data associated with the M primary cores.

In a specific example, the data processing circuitry calculates aderivative of the phase measured in each of the M primary cores and theN redundant cores, and multiplies the phase derivatives for the Mprimary cores by a primary core conversion matrix and the phasederivatives for the N redundant cores by a redundant core conversionmatrix.

In an example application the interferometric measurement system, whenthe optical fiber is inserted in a catheter of a robotic medicalinstrument having motor-operated movements, the data processingcircuitry (1) determines that the detected measurement interferometricpattern data associated with the M primary cores is reliable based onthe comparison, and (2) uses the reliable detected measurementinterferometric pattern data associated with the M primary cores asfeedback for actuating motors to move the robotic medical instrument toa particular shape or location.

In some example embodiments, the number M of primary cores is sufficientto determine twist or temperature associated with the fiber in additionto the primary core fiber bend value.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates mathematical parameters that can be used to quantifycore placement and a response to strain for a six core optical fiber.

FIG. 2 shows a schematic diagram of a first example embodiment of anoptical frequency domain reflectometry (OFDR)-based bend sensing systemthat identifies errors using a redundant measurement of the bend angles.

FIG. 3 is a flowchart diagram for calibrating the optical bend sensingfiber in the first example embodiment.

FIG. 4 is a flowchart diagram for detecting an error in bend sensingsystem of the first example embodiment.

FIG. 5 illustrates mathematical parameters that can be used to quantifycore placement and a response to strain for a six core optical fiberwhere one core is shared for primary and redundant calculations.

FIG. 6 shows an example application of the disclosed technology in asurgical robotic instrument.

DETAILED DESCRIPTION

The following description sets forth specific details, such asparticular embodiments for purposes of explanation and not limitation.But it will be appreciated by one skilled in the art that otherembodiments may be employed apart from these specific details. In someinstances, detailed descriptions of well-known methods, interfaces,circuits, and devices are omitted so as not to obscure the descriptionwith unnecessary detail. Individual blocks are shown in the figurescorresponding to various nodes. Those skilled in the art will appreciatethat the functions of those blocks may be implemented using individualhardware circuits, using software programs and data in conjunction witha suitably programmed digital microprocessor or general purposecomputer, and/or using applications specific integrated circuitry(ASIC), and/or using one or more digital signal processors (DSPs).Software program instructions and data may be stored on anon-transitory, computer-readable storage medium, and when theinstructions are executed by a computer or other suitable processorcontrol, the computer or processor performs the functions associatedwith those instructions.

Overview

A multi-core optical fiber contains to a primary set of optical coresused to provide, in conjunction with an OFDR instrument,interferometric-based measurement of bend angles of the fiber. Inexample embodiments, the multi-core optical fiber also contains a secondset of multiple cores for determining a redundant, interferometric-basedmeasurement of the bend angles of the fiber. The redundant measurementof the fiber bend angles is compared to the primary measurement of thefiber bend angles in order to increase the confidence and reliability inthe primary measurement. The redundant cores are located in differentpositions within the same multi-core fiber than the cores used in theprimary measurement and detect errors caused by incorrect distributedstrain measurements in any of the primary cores and errors caused by anincorrect mapping of distributed strain measurements to bend angles. Inaddition, since the additional cores require partially independentcircuitry from the primary cores, any errors caused by the independentcircuitry can also be detected.

Example multi-core fibers are described below for purposes ofillustration and not limitation. The principles described also apply toa multi-core fiber where multiple primary cores and redundant cores havedifferent relative positions along a length of the optical fiber. Thetechnology applies to a spun fiber where the cores are helically wrappedin order to measure the twist on the fiber. The principles also apply toa multi-core fiber with more than six cores.

First Example Embodiment

FIG. 1 shows a six core optical fiber along with mathematical parametersthat can be used to quantify core placement and a response to strain.Although the term core is used, the technology applies to other types ofwaveguides that can be used in a fiber. Note that different numbers ofcores may be used and that for simplicity all cores are at the sameradius from the center of the multicore fiber. There are three primarymeasurement cores a₁, b₁, and c₁ and three redundant measurement coresa₂, b₂, and c₂. The number of primary measurement cores may begeneralized to a positive integer M, and the number of redundantmeasurement cores may be generalized to a positive integer N. M and Nmay be the same as they are in FIG. 1, i.e., M=3 and N=3, or differentas in FIG. 5, i.e., M=4 and N=2, described below.

A vertical axis through the center of the multi-core fiber passesthrough two of the outer or peripheral cores a₁ and a₂. These outercores a₁ and a₂ are referred to as “reference cores” because severalparameters are expressed relative to these cores a₁ and a₂. So core(s)identified herein with the letter “a” serve as one or more referencecores.

Two parameters describe the position of a core: the radial distance rfrom the fiber center, and an arbitrary angle ϕ measured from thevertical axis intersecting the reference core(s). As the fiber is bent,the amount of bend-induced strain in a given core is directlyproportional to the perpendicular distance d that the core is separatedfrom the bend plane shown as a double-dashed line. This distance d isillustrated in FIG. 1 for the core c₁.

It is helpful to understand how these parameters impact the strainprofile of the fiber when the core strain responses are used together tocalculate the bend strain and axial strain applied to the fiber. Amathematical model is established based on the parameters shown inFIG. 1. Because these parameters can be measured, they can be used toprovide a more accurate recombination of the strain profile of themulti-core optical fiber. It is notable that these parameters need onlybe measured once for a particular multi-core optical fiber and may beused for some or all OFDR subsequent measurements of that samemulti-core optical fiber.

The bend strain B perceived by a core as a result of bending of thefiber is proportional to curvature of the bend and the tangentialdistance d of the core to the bend plane (shown in FIG. 1) in Equation(1) below:B _(n)(z)=αK(z)d _(n)(z)  (1)in which α is a constant, K is the curvature of the fiber, and drepresents the tangential distance of the core from the bend plane. Fromparameters shown in FIG. 1, the tangential distance d can be expressedin terms of the core's position as:d _(n)(z)=r _(n)[sin(ϕ_(n))cos(θ(z))−cos(ϕ_(n))sin(θ(z))]  (2)in which r is the radial distance from the axis of the fiber, ϕrepresents the angle measured from the vertical axis, and θ is a measureof the angle between the bend plane and the vertical axis. Combiningequations (1) and (2) results in:B _(n)(z)=αK(z)r _(n)[sin(ϕ_(n))cos(θ(z))−cos(ϕ_(n))sin(θ(z))]  (3)This expression can be simplified by distributing the curvature term andexpressing as two separate components:B _(n)(z)=αr _(n) └K _(x)(z)sin(ϕ_(n))−K _(y)(z)cos(ϕ_(n))┘  (4)in which K_(x) is the curvature about the horizontal axis (pitch) andK_(y) is the curvature about the vertical axis (yaw).

To a first order, it can also be assumed that the axial strain Aexperienced by the cores is common to all cores within the fiber and isnot dependent on the position of the cores to arrive at the expression:A _(n)(z)=γE(z)  (5)in which γ is a constant, and E represents axial strain. The totalstrain on a core can be written by combining equation(4) and equation(5)in the following expression:ε_(n)(z)=αr _(n) K _(x)(z)sin(ϕ_(n))−αr _(n) K_(y)(z)cos(ϕ_(n))+γE(z)  (6)

Considering the measured strain signals from the three primary cores inthis example fiber embodiment, a matrix relationship can be constructedas follows:

$\begin{matrix}{\begin{bmatrix}{ɛ_{a}(z)} \\{ɛ_{b}(z)} \\{ɛ_{c}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha\; r_{a}{\sin\left( \phi_{a} \right)}} & {{- \alpha}\; r_{a}{\cos\left( \phi_{a} \right)}} & \gamma \\{\alpha\; r_{b}{\sin\left( \phi_{b} \right)}} & {{- \alpha}\; r_{b}{\cos\left( \phi_{b} \right)}} & \gamma \\{\alpha\; r_{c}{\sin\left( \phi_{c} \right)}} & {{- \alpha}\; r_{c}{\cos\left( \phi_{c} \right)}} & \gamma\end{bmatrix}\begin{bmatrix}{K_{x}(z)} \\{K_{y}(z)} \\{E(z)}\end{bmatrix}}} & (7)\end{matrix}$

Equation (7) can be solved for the bend and strain parameters asfollows:

$\begin{matrix}{\begin{bmatrix}{K_{x\; 1}(z)} \\{K_{y\; 1}(z)} \\{E_{1}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha\; r_{a\; 1}{\sin\left( \phi_{a\; 1} \right)}} & {{- \alpha}\; r_{a\; 1}{\cos\left( \phi_{a\; 1} \right)}} & \gamma \\{\alpha\; r_{b\; 1}{\sin\left( \phi_{b\; 1} \right)}} & {{- \alpha}\; r_{b\; 1}{\cos\left( \phi_{b\; 1} \right)}} & \gamma \\{\alpha\; r_{c\; 1}{\sin\left( \phi_{c\; 1} \right)}} & {{- \alpha}\; r_{c\; 1}{\cos\left( \phi_{c\; 1} \right)}} & \gamma\end{bmatrix}^{- 1}\begin{bmatrix}{ɛ_{a\; 1}(z)} \\{ɛ_{b\; 1}(z)} \\{ɛ_{c\; 1}(z)}\end{bmatrix}}} & (8)\end{matrix}$

This expression in equation (8) allows recombination of individualstrain signals of each independent core within the fiber, according tofiber structure variations, and sorting of these signals into strainsthat are applied to the entire multi-core fiber structure. Any number oflinear combinations can be derived from equation (8) to createexpressions that relate the strain response of a core to a component ofthe strain profile.

In a six core multi-core fiber example like that in FIG. 1, equation (8)can be applied twice to two independent triads of cores, i.e., the threeprimary measurement cores a₁, b₁, and c₁ and the three redundantmeasurement cores a₂, b₂, and c₂, shown in equation (9) and equation(10).

$\begin{matrix}{\begin{bmatrix}{K_{x\; 1}(z)} \\{K_{y\; 1}(z)} \\{E_{1}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha\; r_{a\; 1}{\sin\left( \phi_{a\; 1} \right)}} & {{- \alpha}\; r_{a\; 1}{\cos\left( \phi_{a\; 1} \right)}} & \gamma \\{\alpha\; r_{b\; 1}{\sin\left( \phi_{b\; 1} \right)}} & {{- \alpha}\; r_{b\; 1}{\cos\left( \phi_{b\; 1} \right)}} & \gamma \\{\alpha\; r_{c\; 1}{\sin\left( \phi_{c\; 1} \right)}} & {{- \alpha}\; r_{c\; 1}{\cos\left( \phi_{c\; 1} \right)}} & \gamma\end{bmatrix}^{- 1}\begin{bmatrix}{ɛ_{a\; 1}(z)} \\{ɛ_{b\; 1}(z)} \\{ɛ_{c\; 1}(z)}\end{bmatrix}}} & (9) \\{\begin{bmatrix}{K_{x\; 2}(z)} \\{K_{y\; 2}(z)} \\{E_{2}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha\; r_{a\; 2}{\sin\left( \phi_{a\; 2} \right)}} & {{- \alpha}\; r_{a\; 2}{\cos\left( \phi_{a\; 2} \right)}} & \gamma \\{\alpha\; r_{b\; 2}{\sin\left( \phi_{b\; 2} \right)}} & {{- \alpha}\; r_{b\; 2}{\cos\left( \phi_{b\; 2} \right)}} & \gamma \\{\alpha\; r_{c\; 2}{\sin\left( \phi_{c\; 2} \right)}} & {{- \alpha}\; r_{c\; 2}{\cos\left( \phi_{c\; 2} \right)}} & \gamma\end{bmatrix}^{- 1}\begin{bmatrix}{ɛ_{a\; 2}(z)} \\{ɛ_{b\; 2}(z)} \\{ɛ_{c\; 2}(z)}\end{bmatrix}}} & (10)\end{matrix}$

Since both the primary and redundant core triads experience the sameglobal strains, the values K_(x), K_(y), and E calculated for bothtriads should agree with one another. Any disagreement indicates anerror in either the measurement or the assumptions underlying themeasurement. In either case, the current measurement must be assumed tobe flawed, and the bend and strain calculated must be consideredunreliable and a potential hazard.

FIG. 2 shows a schematic diagram of an example embodiment of an opticalfrequency domain reflectometry (OFDR)-based bend sensing system 10 thatuses a six core fiber like that in FIG. 1 to calculate redundant bendangles to detect measurement errors. The OFDR-based bend sensing systemmay be used to calculate redundant bend angles to detect measurementerrors for other multicore fiber configurations.

An OFDR-based distributed strain sensing system 10 includes a lightsource 11, an interferometric interrogator 15, a laser monitor network12, an optical fiber sensor 17, acquisition electronics 18, and a dataprocessor 20. A single OFDR measurement and processing channelcorresponds to one fiber core. During an OFDR measurement, a tunablelight source 11 is swept through a range of optical frequencies. Thislight is split with the use of optical couplers and routed to separateinterferometers. A reference interferometer is part of a laser monitornetwork 12 containing a Hydrogen Cyanide (HCN) gas cell that provides anabsolute wavelength reference throughout the measurement scan. Thereference interferometer within the laser monitor network 12 measuresfluctuations in tuning rate as the light source 11 is scanned through afrequency range.

Measurement interferometric interrogators 15 are connected to respectiveindividual cores in a length of multi-core bend sensing fiber 17. Lightenters the sensing fiber 17 through the measurement arms of the sixinterferometric interrogators referenced generally at 15 correspondingto six core waveguides a₁-c₁ and a₂-c₂ in the fiber 17. Scattered lightfrom each core in the sensing fiber 17 is then interfered with lightthat has traveled along the reference arm of the correspondinginterferometric interrogator 15. Each pairing of an interferometricinterrogator with a core in the multi-core fiber is referred to as anacquisition channel. As the tunable light source 11 is swept, eachchannel is simultaneously measured, and the resulting interferencepattern from each channel is routed to the data acquisition electronics18 adapted for the additional interferometers 15. Each channel isprocessed independently.

A series of optical detectors (e.g., photodiodes) convert the lightsignals from the laser monitor network, gas cell, and the interferencepatterns from each core from the sensing fiber to electrical signals.Processing circuitry in data acquisition unit 18 uses the informationfrom the laser monitor 12 interferometer to resample the detectedinterference patterns of the sensing fiber 17 so that the patternspossess increments constant in optical frequency. This step is amathematical requisite of the Fourier transform operation. Onceresampled, a Fourier transform is performed by the system controllerdata processor 20 to produce a light scatter signal in the temporaldomain. In the temporal domain, the amplitudes of the light scatteringevents are depicted against delay along the length of the fiber. Usingthe distance that light travels in a given increment of time, this delaycan be converted to a measure of length along the sensing fiber. Inother words, the light scatter signal indicates each scattering event asa function of distance along the fiber. The sampling period is referredto as the spatial resolution and is inversely proportional to thefrequency range that the tunable light source 11 is swept through duringthe measurement.

As the fiber is strained, the local light scatters shift as the fiberchanges in physical length. These distortions are highly repeatable.Hence, an OFDR measurement of detected light scatter for the fiber canbe retained in memory that serves as a reference pattern of the fiber inan unstrained state. A subsequently measured scatter signal when thefiber is under strain may then be compared to this reference pattern bythe system controller 20 to gain a measure of shift in delay of thelocal scatters along the length of the sensing fiber. This shift indelay manifests as a continuous, slowly varying optical phase signalwhen compared against the reference scatter pattern. The derivative ofthis optical phase signal is directly proportional to change in physicallength of the sensing core. Change in physical length may be scaled tostrain thereby producing a continuous measurement of strain along thesensing fiber.

The data processor 22 coupled to the system controller 20 extractsparameters 24 relating to the actual physical configuration of the coresa₁-c₁ and a₂-c₂ in fiber 17 that are used to calibrate or otherwisecompensate the OFDR measurements to account for the variations betweenthe actual optical core configuration and an optimal optical coreconfiguration. The mathematical model described in detail above is firstestablished that depicts parameters that describe variations from anoptimal multi-core fiber configuration, where the term “optimal”includes known and unknown configurations. Calibration parameters arethen defined that compensate for variation in the physical properties ofthe optical cores within the multi-core fiber.

FIG. 3 is a flowchart diagram for calibrating a six core optical bendsensing fiber like that shown in FIG. 1. Initially, the multi-core fiberis placed in a straight line, unstrained configuration, and an OFDRmeasurement is performed (step S1) as described above, and the resultingreference state parameters are stored (step S2). The multi-core fiber isthen configured in a known configuration such as in a flat plane, in ahelical bend (e.g., a screw), or in any known configuration (step S3).In a non-limiting example, the multi-core fiber is configured in a flatplane (see the spiral shape shown to the right of step S3) to calculatethe relative geometry between the cores in the fiber (step S4). Thefiber is then configured into a known bend position (step S5), and abend gain is calculated that provides amplitude values of the coregeometry (step S6). The fiber is placed under tension (step S7), and atension response for each core calculated (step S8). The values neededto populate the matrix in equation (8) above, which describes theresponse of the six cores to bend and strain are then determined, andthe matrix is inverted (step S9).

FIG. 4 is a flowchart diagram, carried out by the system controller dataprocessor 20 in example embodiments, for detecting an error in bendsensing system using a calibrated bend sensing fiber having M primarycores and N redundant. The flowchart process steps can be executed byany processor (e.g., FPGA, CPU, GPU, ASIC, etc.) including acquisitionelectronics 18 or data processor 22. Initially, the already-calibratedbend sensing fiber (e.g., see FIG. 3) is placed as desired for bendsensing, and OFDR measurements are obtained for each of the M+N cores (Mprimary cores and N redundant cores) (step S20). The data processor 22tracks the optical phase signal for each core determined from these OFDRmeasurements as compared to the calibrated reference OFDR patterns foreach corresponding core for this fiber as obtained for example followingthe example fiber calibration procedures from FIG. 3 (step S21). Each ofthe tracked optical phase signals is a measure of shift in delay of thereflections, e.g., local scatters or Bragg fiber grating reflections,along the length of its respective core in the sensing fiber. Thederivative of this optical phase signal is calculated for each of thecores (step S22), which is directly proportional to change in physicallength of its respective core. Each of the six phase derivatives ismultiplied by the conversion matrix from equation (8) to determine theapplied bend and strain (step S23).

Then the primary core OFDR measurements and redundant core OFDRmeasurements are compared to determine the reliability of themeasurement (step S24). If the difference between the primary andredundant core OFDR measurements differ by more than a predeterminedamount, then the OFDR measurements are labeled or otherwise indicated asunreliable and/or one or more the following actions is taken orinitiated: generate a fault signal for display, stop operation of thesystem or machine associated with the bend sensing fiber, generate analarm, and/or take some other precautionary or protection action (stepS25).

Second Example Embodiment

The technology can also be applied to a multi-core fiber where more thanthree primary measurement cores are used. Additional primary measurementcores can be reused for the redundant bend calculation. This coreconfiguration is useful for extracting additional parameters, such asthe twist on the fiber or the temperature change along the fiber.

FIG. 5 illustrates mathematical parameters that can be used to quantifycore placement and a response to strain for a six core optical fiber. Inthis example embodiment, core di is included as one of the primary coresin order to calculate the twist on a helical multi-core fiber, oneexample of which is described in U.S. provisional patent applicationSer. No. 62/347,704, filed on Jun. 9, 2016, which is incorporated hereinby reference. The same core is also used as a redundant core a₂. Thus,in this example, M=4 and N=2.

The inclusion of core di in the primary calculation allows the twist onthe fiber to be calculated. For moderate levels of twist applied to afiber (e.g., 100 degrees/meter), a first order term can be used to modelstrain induced by torque. Twist strain R_(n)(z) is then expressed interms of the core position as follows:R _(n)(z)=βr _(n) ²Φ(z)  (11)in which β is a constant, and Φ is the amount the fiber has twisted(roll), per unit of length. The total strain on a core can be written bycombining equation (4), equation (5), and equation (11) in the followingexpression:ε_(n)(z)=αr _(n) K _(x)(z)sin(ϕ_(n))−αr _(n) K_(y)(z)cos(ϕ_(n))+γE(z)+βr _(n) ²Φ(z)  (12)

Considering the measured strain signals from the four primary cores inthis example embodiment, a matrix relationship can be constructed asfollows:

$\begin{matrix}{\begin{bmatrix}{ɛ_{a\; 1}(z)} \\{ɛ_{b\; 1}(z)} \\{ɛ_{c\; 1}(z)} \\{ɛ_{d\; 1}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha\; r_{a}{\sin\left( \phi_{a} \right)}} & {{- \alpha}\; r_{a}{\cos\left( \phi_{a} \right)}} & \gamma & {\beta\; r_{a\; 1}^{2}} \\{\alpha\; r_{b}{\sin\left( \phi_{b} \right)}} & {{- \alpha}\; r_{b}{\cos\left( \phi_{b} \right)}} & \gamma & {\beta\; r_{b\; 1}^{2}} \\{\alpha\; r_{c}{\sin\left( \phi_{c} \right)}} & {{- \alpha}\; r_{c}{\cos\left( \phi_{c} \right)}} & \gamma & {\beta\; r_{c\; 1}^{2}} \\{\alpha\; r_{d}{\sin\left( \phi_{d} \right)}} & {{- \alpha}\; r_{d}{\cos\left( \phi_{d} \right)}} & \gamma & {\beta\; r_{d\; 1}^{2}}\end{bmatrix}^{- 1}\begin{bmatrix}{K_{x\; 1}(z)} \\{K_{y\; 1}(z)} \\{E_{1}(z)} \\{\Phi_{1}(z)}\end{bmatrix}}} & (13)\end{matrix}$

Equation (13) can be solved for the bend (K_(x), K_(y)), strain (E), andtwist (Φ) parameters as follows:

$\begin{matrix}{\begin{bmatrix}{K_{x\; 1}(z)} \\{K_{y\; 1}(z)} \\{E_{1}(z)} \\{\phi_{1}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha\; r_{a}{\sin\left( \phi_{a} \right)}} & {{- \alpha}\; r_{a}{\cos\left( \phi_{a} \right)}} & \gamma & {\beta\; r_{a\; 1}^{2}} \\{\alpha\; r_{b}{\sin\left( \phi_{b} \right)}} & {{- \alpha}\; r_{b}{\cos\left( \phi_{b} \right)}} & \gamma & {\beta\; r_{b\; 1}^{2}} \\{\alpha\; r_{c}{\sin\left( \phi_{c} \right)}} & {{- \alpha}\; r_{c}{\cos\left( \phi_{c} \right)}} & \gamma & {\beta\; r_{c\; 1}^{2}} \\{\alpha\; r_{d}{\sin\left( \phi_{d} \right)}} & {{- \alpha}\; r_{d}{\cos\left( \phi_{d} \right)}} & \gamma & {\beta\; r_{d\; 1}^{2}}\end{bmatrix}^{- 1}\begin{bmatrix}{ɛ_{a\; 1}(z)} \\{ɛ_{b\; 1}(z)} \\{ɛ_{c\; 1}(z)} \\{ɛ_{d\; 1}(z)}\end{bmatrix}}} & (14)\end{matrix}$

This expression in equation (14) allows recombination of individualstrain signals of each independent core within the fiber, according tofiber structure variations, and sorting of these signals into strainsthat are applied to the entire multi-core fiber structure. Any number oflinear combinations can be derived from equation (14) to createexpressions that relate the strain response of a core to a component ofthe strain profile.

The redundant bend calculation for this embodiment can be calculatedusing equation (10). Since both the primary and redundant coresexperience the same global strains, the values K_(x), K_(y), and Ecalculated for both the primary and redundant cores should agree withone another. Any disagreement indicates an error in either the primarycore OFDR measurements or the assumptions underlying the measurements.In either case, the current primary core OFDR measurements should beassumed flawed, and the bend and strain calculated therefrom consideredunreliable and a potential hazard.

FIG. 6 is a simplified diagram of a medical instrument system 200according to example embodiments that benefits from the technologydescribed above. In some embodiments, medical instrument system 200 maybe used in an image-guided medical procedure performed with teleoperatedmedical system. In some examples, medical instrument system 200 may beused for non-teleoperational exploratory procedures or in proceduresinvolving traditional manually operated medical instruments, such asendoscopy. Optionally, medical instrument system 200 may be used togather (i.e., measure) a set of data points corresponding to locationswithin anatomic passageways of a patient.

Medical instrument system 200 includes elongate device 202 coupled to amotor drive unit 204. Elongate device 202 includes a flexible body 216having proximal end 217 and distal end or tip portion 218. In someembodiments, flexible body 216 has an approximately 3 mm outer diameter.Other flexible body outer diameters may be larger or smaller.

Medical instrument system 200 further includes a tracking system 230 fordetermining the position, orientation, speed, velocity, pose, and/orshape of a catheter tip at distal end 218 and/or of one or more segments224 along flexible body 216 using one or more sensors and/or imagingdevices. One catheter/lumen is labeled 222 and includes optical fiber17. The entire length of flexible body 216, between distal end 218 andproximal end 217, may be effectively divided into segments 224. Thetracking system 230 may be implemented as hardware, firmware, softwareor a combination thereof which interact with or are otherwise executedby one or more computer processors.

The OFDR system 10 and tracking system 230 track the bend of the distalend 218 and/or one or more of the segments 224 using a sensing fiber 17contained in catheter/lumen 222. In one example embodiment, the sensingfiber has a diameter of approximately 200 μm. In other embodiments, thedimensions may be larger or smaller. The sensing fiber forms a fiberoptic bend sensor that the OFDR system 10 measures as described aboveand provides feedback about the amount of bend at the distal end 218 tothe tracking system 230. The tracking system 230 uses the bend data tocontrol the motor drive unit 204 to position pull wires in the fiber.Because accurate control of the tip 218 of the catheter/lumen depends onaccurate bend data from the OFDR system 10, any inaccurate data from thebend sensing fiber/OFDR system should be flagged as inaccurate orunreliable so that the tracking system 230 can make correct decisionsabout the current OFDR data rather than moving the tip 218 to anincorrect location. The redundant core technology described aboveensures accurate and reliable feedback.

Flexible body 216 includes a channel 226 sized and shaped to receive amedical instrument. In some embodiments, medical instrument may be usedfor procedures such as surgery, biopsy, ablation, illumination,irrigation, or suction. The medical instrument may include, for example,image capture probes, biopsy instruments, laser ablation fibers, and/orother surgical, diagnostic, or therapeutic tools. Medical tools mayinclude end effectors having a single working member such as a scalpel,a blunt blade, an optical fiber, an electrode, and/or the like. Otherend effectors may include, for example, forceps, graspers, scissors,clip appliers, and/or the like. Other end effectors may further includeelectrically activated end effectors such as electrosurgical electrodes,transducers, sensors, and/or the like.

The medical instrument channel 226 may additionally house cables,linkages, or other actuation controls (not shown) that extend betweenits proximal and distal ends to controllably adjust the bend distal endof medical instrument. Steerable instruments are described in detail inU.S. Pat. No. 7,316,681 and U.S. patent application Ser. No. 12/286,644,which are incorporated by reference herein in their entireties.

Flexible body 216 may also house cables, linkages, or other steeringcontrols (not shown) that extend between drive unit 204 and distal end218 to controllably bend distal end 218 as shown, for example, by brokendashed line depictions 219 of distal end 218. In some examples, at leastfour cables are used to provide independent “up-down” steering tocontrol a pitch of distal end 218 and “left-right” steering to control ayaw of distal end 281. Steerable catheters are described in detail inU.S. patent application Ser. No. 13/274,208, which is incorporated byreference herein in its entirety. In embodiments in which medicalinstrument system 200 is actuated by a teleoperational assembly 234,drive unit 204 may include drive inputs that removably couple to andreceive power from drive elements, such as actuators, of theteleoperational assembly. The drive unit 204 and tracking system 230form a drive unit control loop that uses bend data measured by the OFDRsystem 10 from the fiber 17 to determine the current position,orientation, shape, etc. of the tip 218 of the catheter (e.g., the tipmight be bent 30 degrees left and 10 degrees up). The drive unit controlloop receives a command, e.g., from a user like a surgeon, to move thetip 218 to a new position and uses the difference between the tip'scurrent position determined by the OFDR measurements and the commandedposition to determine which control wires to actuate by drive unit 204to move the catheter tip 218 to the commanded position. The drive unitcontrol loop monitors the bend data from the fiber 17/OFDR system 10 toensure correct motion to the commanded position as the pull wires aremanipulated by drive unit 204.

In some embodiments, medical instrument system 200 may include grippingfeatures, manual actuators, or other components for manually controllingthe motion of medical instrument system 200. Elongate device 202 may besteerable or, alternatively, the system may be non-steerable with nointegrated mechanism for operator control of the bending of distal end218. In some examples, one or more lumens, through which medicalinstruments can be deployed and used at a target surgical location, aredefined in the walls of flexible body 216.

The information from the OFDR system 10 and the tracking system 230 maybe sent to a navigation system 232 where it may be combined withinformation from visualization system 231 and/or the preoperativelyobtained models to provide the physician, clinician, or surgeon or otheroperator with real-time position information. In some examples, thereal-time position information may be displayed on display system 233for use in the control of medical instrument system 200. In someexamples, control system 116 of FIG. 1 may utilize position informationfrom the fiber as measured by the OFDR system 10 as feedback forpositioning medical instrument system 200. Various systems for usingfiber optic sensors to register and display a surgical instrument withsurgical images are provided in U.S. patent application Ser. No.13/107,562, filed May 13, 2011,” which is incorporated by referenceherein in its entirety.

The technology described above enables a single bend sensing fiber to beused as a bend encoder for a surgical system where any inaccuracy in thebend output must be detected and reported to the system controller. Thistechnology may be used beyond surgical systems to any systems thatrequire inaccurate bend output to be detected for purposes of safety orcorrect operation). Using a single bend sensing fiber has severaladvantages over using a secondary bend encoder to detect inaccuratemeasurements: it is simpler to design, cheaper to manufacture, andrequires less space than multiple encoders. Using a single fiber toperform both primary and redundant measurements may also be moreaccurate than using multiple encoders since the single fiber isguaranteed to experience the same global bends and strains, whereasmultiple encoders exist in distinct spaces within the robot, which maycause them to experience different bends and strains.

Those skilled in the art will appreciate that figures in thisapplication may represent conceptual views of illustrative circuitry orother functional units. Similarly, it will be appreciated that flowcharts, state transition diagrams, pseudo-code, and the like typicallyrepresent various processes which may be substantially represented incomputer-readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various illustrated elements may be providedthrough the use of hardware such as circuit hardware and/or hardwarecapable of executing software in the form of coded instructions storedon computer-readable medium. Thus, such functions and illustratedfunctional blocks are to be understood as being eitherhardware-implemented and/or computer-implemented, and thus,machine-implemented.

In terms of hardware implementation, the functional blocks may includeor encompass, without limitation, a digital signal processor (DSP)hardware, a reduced instruction set processor, hardware (e.g., digitalor analog) circuitry including but not limited to application specificintegrated circuit(s) (ASIC) and/or field programmable gate array(s)(FPGA(s)), and (where appropriate) state machines capable of performingsuch functions.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer, processor, and controller may be employedinterchangeably. When provided by a computer, processor, or controller,the functions may be provided by a single dedicated computer orprocessor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed. Moreover, theterm “processor” or “controller” also refers to other hardware capableof performing such functions and/or executing software, such as theexample hardware recited above.

Whenever it is described in this document that a given item is presentin “some embodiments,” “various embodiments,” “certain embodiments,”“certain example embodiments, “some example embodiments,” “an exemplaryembodiment,” or whenever any other similar language is used, it shouldbe understood that the given item is present in at least one embodiment,though is not necessarily present in all embodiments. Consistent withthe foregoing, whenever it is described in this document that an action“may,” “can,” or “could” be performed, that a feature, element, orcomponent “may,” “can,” or “could” be included in or is applicable to agiven context, that a given item “may,” “can,” or “could” possess agiven attribute, or whenever any similar phrase involving the term“may,” “can,” or “could” is used, it should be understood that the givenaction, feature, element, component, attribute, etc. is present in atleast one embodiment, though is not necessarily present in allembodiments. Terms and phrases used in this document, and variationsthereof, unless otherwise expressly stated, should be construed asopen-ended rather than limiting. As examples of the foregoing: “and/or”includes any and all combinations of one or more of the associatedlisted items (e.g., a and/or b means a, b, or a and b); the singularforms “a”, “an” and “the” should be read as meaning “at least one,” “oneor more,” or the like; the term “example” is used provide examples ofthe subject under discussion, not an exhaustive or limiting listthereof; the terms “comprise” and “include” (and other conjugations andother variations thereof) specify the presence of the associated listeditems but do not preclude the presence or addition of one or more otheritems; and if an item is described as “optional,” such descriptionshould not be understood to indicate that other items are also notoptional.

As used herein, the term “non-transitory computer-readable storagemedium” includes a register, a cache memory, a ROM, a semiconductormemory device (such as a D-RAM, S-RAM, or other RAM), a magnetic mediumsuch as a flash memory, a hard disk, a magneto-optical medium, anoptical medium such as a CD-ROM, a DVD, or Blu-Ray Disc, or other typeof device for non-transitory electronic data storage. The term“non-transitory computer-readable storage medium” does not include atransitory, propagating electromagnetic signal.

Although various embodiments have been shown and described in detail,the claims are not limited to any particular embodiment or example. Thetechnology fully encompasses other embodiments which may become apparentto those skilled in the art. None of the above description should beread as implying that any particular element, step, range, or functionis essential such that it must be included in the claims scope. Thescope of patented subject matter is defined only by the claims. Theextent of legal protection is defined by the words recited in the claimsand their equivalents. All structural and functional equivalents to theelements of the above-described preferred embodiment that are known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the present claims.Moreover, it is not necessary for a device or method to address each andevery problem sought to be solved by the technology described, for it tobe encompassed by the present claims. No claim is intended to invoke 35USC § 112(f) unless the words “means for” or “step for” are used.Furthermore, no embodiment, feature, component, or step in thisspecification is intended to be dedicated to the public regardless ofwhether the embodiment, feature, component, or step is recited in theclaims.

The invention claimed is:
 1. An interferometric measurement system formeasuring an optical fiber including at least four primary cores and atleast two redundant cores, the interferometric measurement systemcomprising: interferometric detection circuitry configured to detectmeasurement interferometric pattern data associated with the at leastfour primary cores and the at least two redundant cores when the opticalfiber is placed into a sensing position; and data processing circuitryconfigured to: calculate primary-core values based on a predeterminedgeometry of the at least four primary cores and of the at least tworedundant cores and on the detected measurement interferometric patterndata associated with the at least four primary cores, the primary-corevalues including primary-core values of first and second fiber bendparameters, of a fiber strain parameter, and of a fiber twist ortemperature parameter for the optical fiber; calculate redundant valuesbased on a predetermined geometry of a first primary core of the atleast four primary cores and of the at least two redundant cores, and onthe detected measurement interferometric pattern data associated withthe first primary core and the at least two redundant cores, theredundant values including redundant values of the first and secondfiber bend parameters and of the fiber strain parameter; perform acomparison of the primary-core values and the redundant values of thefirst and second fiber bend parameters and of the fiber strainparameter; determine whether the primary-core values of the first andsecond fiber bend parameters and of the fiber strain parameter arereliable or unreliable based on the comparison, and generate a signal inresponse to determination that the primary-core values of the first andsecond fiber bend parameters and the fiber strain parameter areunreliable.
 2. The interferometric measurement system in claim 1,wherein the signal represents an error comprising one or more of thefollowing: (a) an error in operation of optical sensing and processingcircuitry, (b) an error in operation of electronic sensing andprocessing circuitry, (c) an error in a connection with the opticalfiber, (d) an error in calibration data determined for the opticalfiber, (e) an error caused by a force experienced by the fiber that isotherwise not compensated for by the data processing circuitry, and (f)an error in processing or execution of algorithms resulting inmiscalculation of bend value.
 3. The interferometric measurement systemin claim 1, wherein the at least four primary cores include exactly fourprimary cores and the at least two redundant cores include exactly tworedundant cores.
 4. The interferometric measurement system in claim 1,wherein the data processing circuitry is configured to: calculate phasederivatives by calculating a derivative of a phase measured in each ofthe at least four primary cores and the at least two redundant cores,multiply the phase derivatives for the at least four primary cores by aprimary core conversion matrix, and multiply the phase derivatives forthe at least two redundant cores and the first primary core by aredundant core conversion matrix.
 5. The interferometric measurementsystem in claim 1, wherein the data processing circuitry is configuredto determine the primary-core values of the first and second fiber bendparameters and the fiber strain parameter to be unreliable when adifference between the primary-core values and the redundant values ofthe first and second fiber bend parameters and the fiber strainparameter exceeds a predetermined threshold.
 6. The interferometricmeasurement system in claim 1, wherein when the optical fiber isinserted in a catheter of a robotic medical instrument havingmotor-operated movements, the data processing circuitry is configuredto: use the reliable primary-core values of the first and second fiberbend parameters and the fiber strain parameter as feedback for actuatingmotors to move the robotic medical instrument to a particular shape orlocation.
 7. A method for measuring an optical fiber including at leastfour primary cores and at least two redundant cores, the methodcomprising: detecting, using interferometric detection circuitry,measurement interferometric pattern data associated with the at leastfour primary cores and the at least two redundant cores when the opticalfiber is placed in a sensing position; calculating, using dataprocessing circuitry, at least four primly-core values comprisingprimary-core values of first and second fiber bend parameters, a fiberstrain parameter, and a fiber twist or temperature parameter of theoptical fiber based on a predetermined geometry of the at least fourprimary cores and the detected measurement interferometric pattern dataassociated with the at least four primary cores; calculating, using thedata processing circuitry, redundant values of the first and secondfiber bend parameters and the fiber strain parameter for the opticalfiber based on a predetermined geometry of a first primary core of theat least four primary cores and the at least two redundant cores and thedetected measurement interferometric pattern data associated with thefirst primary core and the at least two redundant cores; performing acomparison of the primary-core values and the redundant values of thefirst and second fiber bend parameters and the fiber strain parameter;determining whether the primary-core values of the first and secondfiber bend parameters and the fiber strain parameter are reliable orunreliable based on the comparison; and generating a signal in responseto a determination that the calculated primary-core values of the firstand second fiber bend parameters and the fiber strain parameter areunreliable.
 8. The method in claim 7, wherein the at least four primarycores include exactly four primary cores and the at least two redundantcores include exactly two redundant cores.
 9. The method in claim 7,further comprising: calculating phase derivatives by calculating aderivative of a phase measured in each of the at least four primarycores and the at least two redundant cores, multiplying the phasederivatives for the at least four primary cores by a primary coreconversion matrix, and multiply the phase derivatives for the at leasttwo redundant cores and the first primary core by a redundant coreconversion matrix.
 10. The method in claim 7, further comprising:determining the primary-core values of the first and second fiber bendparameters and the fiber strain parameter to be unreliable when adifference between the primary-core values and the redundant values ofthe first and second fiber bend parameters and the fiber strainparameter exceeds a predetermined threshold.
 11. The method in claim 7,wherein when the optical fiber is inserted in a catheter of a roboticmedical instrument having motor-operated movements, the method furthercomprising: using the reliable primary-core values of the first andsecond fiber bend parameters and the fiber strain parameter as feedbackfor actuating motors to move the robotic medical instrument to aparticular shape or location.
 12. A computer-readable medium storinginstructions for controlling operation of one or more hardwareprocessors to determine shape parameters of an optical fiber includingat least four primary cores and at least two redundant cores, based onmeasurement interferometric pattern data associated with the at leastfour primary cores and the at least two redundant cores when the opticalfiber is placed in a sensing-position, wherein the instructions, whenexecuted, cause the one or more hardware processors to performoperations comprising: calculating at least four primary-core valuesthat include primary-core values of first and second fiber bend valueparameters, a fiber strain parameter and a fiber twist or temperatureparameter for the optical fiber based on a predetermined geometry of theat least four primary cores and the measurement interferometric patterndata associated with the at least four primary cores; calculatingredundant values of at least the first and second fiber bend parametersand the fiber strain parameter for the optical fiber based on apredetermined geometry of a first primary core of the at least fourprimary cores and the at least two redundant cores and the measurementinterferometric pattern data associated with the one of the at leastfour primary cores and the at least two redundant cores; performing acomparison of the primary-core values and the redundant values of thefirst and second fiber bend parameters and the fiber strain parameter;determining whether the calculated primary-core values of the first andsecond fiber bend parameters and the fiber strain parameter are reliableor unreliable based on the comparison; and generating a signal inresponse to a determination that the calculated primary-core values ofthe first and second fiber bend parameters and the fiber strainparameter are unreliable.
 13. The computer-readable medium of claim 12,wherein the operations further comprise: calculating phase derivativesby calculating a derivative of a phase measured in each of the at leastfour primary cores and the at least two redundant cores, and multiplyingthe phase derivatives for the at least four primary cores by a primarycore conversion matrix and the phase derivatives for the at least tworedundant cores and the first primary core by a redundant coreconversion matrix.
 14. The computer-readable medium of claim 12, whereindetermining whether the primary-core values of the first and secondfiber bend parameters and the fiber strain parameter are reliable orunreliable based on the comparison comprises: determining unreliabilityof the primary-core values of the first and second fiber bend parametersand the fiber strain parameter when a difference between theprimary-core values and the redundant values of the first and secondfiber bend parameters and the fiber strain parameter exceeds apredetermined threshold.
 15. The computer-readable medium of claim 12,wherein when the optical fiber is inserted in a catheter of a roboticmedical instrument having motor-operated movements, and wherein theoperations further comprise: using the reliable primary-core values ofthe first and second fiber bend parameters and the fiber strainparameter as feedback for actuating motors to move the robotic medicalinstrument to a particular shape or location.